Laminated boards for printed circuits are generally manufactured by layering a copper foil on resin-impregnated fabric or so-called prepreg, and then pressing them between hot plates. The resin initially reduces in viscosity to a liquid state, and then gradually hardens. For uniform temprature and pressure distribution, cushion materials capable of enduring repeated use under high temperatures and high pressures are installed on the platens. On the surface of said heat-resisting cushion material, airtightening processing is usually performed for the purpose of preventing the fibrous materials forming the cushion material from falling therefrom or for improving the applicability of sucking force of a vacuum conveying machine.
In order to form the airtight layer on the surface as mentioned above, it is known in Patent Laid-open Gazette No. 128544/1982 to laminate thermoplastic fibers on the surface of the cushion material and to cause the thermoplastic layer to melt and form a film by means of hot rolls and to coat a resin thereon having a good releasing property. Alternatively, it is also known to bond a metallic plate, such as, a copper foil, to the surface of the cushion material by means of a thermosetting resin adhesive (epoxy reisn, etc.).
However, in the first ease, thermoplastic fibers are melted to form a sheet, so that it lacks flexibility, prone to cracks under a bending stress, and functions inadequately as a surface layer. Also, it takes a long time to form an airtight layer and it is problematic in productivity.
On the other hand, in the second case, although it is free from the defects as mentioned before, when the bonding takes place, a hardening-contraction takes place which is peculiar to thermosetting resins, which may remain as a residual stress. Therefore, during use, the contraction of the cushion material as a whole progresses by small degress as the heating, pressing, cooling, releasing steps are repeated, which may cause the resultant products to become defective. Furthermore, thermosetting resins are liable to deteriorate easily; the metallic foils may easily fall off due to the difference in coefficients of heat elongation. In addition, since metallic plates may not be incinerated, incineration processing may not be employed so as to dispose the used cushion materials economically.